The present invention relates, in general, to a high electric field gradient accelerator for particles, and more particularly to a pulsed microwave driven electron accelerator which may be used in high energy physics, or may be operated at a reduced electric field gradient to yield a compact, high average current intermediate energy accelerator.
The direct application of pulse power technology to particle acceleration has been studied for several years using single pulses. Modest successes have been reported and operating capabilities, for various accelerator configurations, defined experimentally and theoretically. Most of this work has been centered on the acceleration of ions in high current relativistic electron beams and is not readily scalable to the needs of high energy physics, although it does have potential applications in other areas.
Contemporary pulse power technology has its origins in the work of J. C. Martin who, in about 1962, used a Marx generator to charge a transmission line which was subsequently switched into a vacuum diode. The operating levels (.about.500 kV, 50 kA, 50 nsec.) achieved were comparable to or exceed most of the specifications for a source of interest for high energy particle accelerator development today, but was capable only of single shot operation. In the Marx generator the primary energy source, a bank of capacitors, was charged from a D.C. supply. The capacitors were then switched into a series configuration having an output capacitance of C/n and an output voltage of nV, where C is the capacitance of each capacitor, n the number of stages, and V the charging voltage. The output from the Marx generator was used to charge a transmission line which in turn was switched into a vacuum diode. Typically, the Marx generator charges the line in a time of the order of 500 nsec.
To move beyond the single shot pulse generator capabilities indicated above required the use of magnetic switching technology. In a magnetically switched pulse generator a repetitively pulsed source feeds a series of LC cascaded circuits in which the inductors are saturable reactors. Prior to use the cores are reset to their remnant magnetization state in the reverse sense to that to be used for the pulse generation. As the first inductor L.sub.n saturates and the first capacitor C.sub.n is fully charged, C.sub.n+1 starts to charge. The pulse compression occurs in successive stages as a result of making the charging times for successive capacitors, and the corresponding saturation times for the inductors, smaller. The charging times scale is the square root of the ratio of the saturated inductances of successive saturable inductors. Thus magnetic switching, which really is magnetic pulse compression, may be used to replace the switch used in the pulse power transmission line. The final capacitor in the last stage is the conventional pulse line or Blumlein line which is used in single shot pulse power systems. An increase of about three can be obtained in the final output voltage pulse through the use of a three to one non-saturating step up transformer.
Most of the work to date in the area of collective acceleration of charged particles in high power beams has centered on the acceleration of positive ions, at moderately high field gradients, from a low energy injector. All of the techniques rely on waves carried on the beam, either as eigenmodes of an unneutralized beam in an evacuated drift tube or by solitary waves propagating at the beam head. In both cases the maximum phase velocity is the beam velocity and the maximum achievable ion energy will not approach energies of interest for high energy physics unless ways can be found to drive the wave phase velocity to values higher than the electron drift velocity.
High power microwave generation using intense relativistic electron beams started in the late 60's and was first reported by J. A. Nation, Applied Physics Letters 17, p. 491 (1970). Since that time there has been an ever increasing effort in this area.
In broad terms there are two regimes of interest for high power microwave generation. These are divided by regime according to the value of the ratio of the beam current to the limiting current, i.e., whether the beam current is greater than or less than the limiting current for the device. The limiting current phenomenon arises from the potential depression in the drift tube caused by the beam space charge. The potential depression decreases the electron velocity below its injection value and hence increases the beam density, which in turn increases the potential depression. The system is stable only for the regime in which the currents are less than the limiting value for the device being used.
Existing high energy accelerators have involved the use of complex structures within the accelerator cavity, and such devices have encountered difficulties since their high electric fields near the cavity walls can lead to breakdowns in the accelerator system. Further, the complex structures cause the particles to deviate from a straight-line path, resulting in high radiation losses. The existing accelerators also are not suitable for ultra-high power microwave drives, because the filling time for the acceleration cavity is too long for the length of the pulses that can be produced by modern pulse generators.